Characterization of the encystment-inducing activity of extracellular fluid from encysting vegetative cells in the terrestrial ciliated protozoa Colpoda cucullus

Abstract

The terrestrial ciliated protozoan Colpoda cucullus transforms into a dehydration-resistant cyst upon sensing adverse signals. Recently, we identified a pheromone secreted by encysting vegetative cells of C. cucullus, termed “encystment-inducing pheromone” (EnIP), and characterized its properties. Overpopulated vegetative cells spontaneously encysted in ion-free ultrapure water without external stimuli. The external solution of encysting cells markedly induced encystment in vegetative cells under low cell density conditions, suggesting that EnIP, secreted by encysting cells into the external solution, induced encystment in vegetative cells. Further investigations revealed the following properties of EnIP: 1) EnIP retains encystment-inducing activity even in the presence of EGTA. 2) EnIP exhibits a concentration-dependent encystment effect. 3) EnIP is released within hours at high cell density. 4) EnIP is released by exocytosis. 5) EnIP loses its activity after 1–2 days. 6) EnIP is inactivated by heating and treatment with proteolytic enzymes. 7) The nominal molecular weight of EnIP was estimated to range between 10 and 100 kDa. These findings lead to the conclusion that encystment-induced C. cucullus vegetative cells secrete EnIP, a protein pheromone, with encystment-inducing activity for other cells. This study contributes to understanding microbial communication and reveals a novel mechanism for protist survival in harsh environments.